1. Field of the Invention
This invention relates generally to a driving method for display elements, light valves, etc., and more particularly it relates to a driving method for display elements that use a liquid crystal substance.
2. Related Art
The tristable switching of antiferroelectric liquid crystal is expected to solve some of the problems inherent in prior art surface stabilized ferroelectric liquid crystal (SSFLC), and its research is actively going forward. (Refer to A. D. L. Chandani, et al.: Jpn. J. Appl. Pys., 27, L729 (1988) and A. D. L. Chandani, et al.: Jpn. J. Appl. Phys., 28, L1265 (1988).)
The main features of tristable switching are:
1) Antiferroelectric-ferroelectric phase transition due to voltage application has a steep threshold characteristic with respect to DC voltage (FIG. 33). PA1 2) Antiferroelectric-ferroelectric phase transition is accompanied by a wide optical hysteresis, and the selected state can be maintained as long as a bias voltage is applied after an antiferroelectric phase or a ferroelectric phase is selected. PA1 3) The two orientation states in an electric field-induced ferroelectric phase can be made optically equivalent. PA1 4) Since polarization of the electric charge in the liquid crystal substance can be prevented, there is no deterioration over time of the electro-optical characteristic such as is seen in SSFLC. PA1 a) a voltage pulse whose absolute value is less than the threshold value if the orientation state to be selected is an antiferroelectric phase; or PA1 b) a voltage pulse whose absolute value is larger than the threshold value if the orientation state to be selected is a ferroelectric phase;
By taking advantage of these characteristics, time-sharing addressing is possible in a simple matrix with no restriction on the duty ratio. Examples of previously known driving methods are noted in M. Yamawaki, et al., Digest of Japan Display '89, p. 26 (1989) (FIG. 30). In FIG. 30, V.sub.t and V.sub.d are the voltage waveforms supplied to the scanning electrodes and signal electrodes, respectively, and V.sub.LC is a composite waveform applied to the liquid crystal layer V.sub.LC =V.sub.t -V.sub.d. In this driving method, frame F(+) on which a positive polarity voltage is applied and the subsequent negative polarity frame F(-) are a pair.
The principle of display by means of this driving method is explained using FIGS. 32A and 32B. Referring if FIG. 32A, optical axis OA in the antiferroelectric phase is perpendicular to the smectic layer. As shown in FIG. 32B, when a cell comprising liquid crystal 6 sandwiched between two glass substrates 1, 2 on which transparent electrodes 4, 5 and alignment films 9, 10 are formed is disposed between two polarizers 11, 12 whose polarization axes are perpendicular to each other such that optical axis OA is parallel to one of the polarization axes, the element goes to a light-blocking condition (tentatively OFF). Even if the voltage waveform in frame F'(+) or F'(-) in FIG. 30 is applied on this condition, as long as .vertline.V.sub.W2 .vertline.&lt;.vertline.V(A-F)t.vertline. (see FIG. 33), the light transmittance changes very little and the OFF condition can be maintained. In case of which the voltage waveform of F(+) or F(-) in FIG. 30 is applied, the liquid crystal will respond if .vertline.V.sub.W1 .vertline.&gt;.vertline.V(A-F)s.vertline., and change to ferroelectric phase(+) or ferroelectric phase (-). Ferroelectric phase(+ ) and ferroelectric phase (-), have the respective optical axes OF(+) and OF(-) and spontaneous polarizations Ps(+) and Ps(-). Since the optical axes form angle .theta.(+) or .theta.(-) with the polarization axis, a light transmission condition (tentatively ON) is set. Since angles .theta.(+) and .theta.(-) are equal, they can both be treated as being optically equivalent.
However, the prior art driving method has the two problems explained below.
One problem concerns the stability of the antiferroelectric phase. The antiferroelectric phase generally has a steep threshold characteristic with respect to DC voltage. Even if a single-polarity bias voltage is applied during the non-selection period (T.sub.22 in the figure) after the antiferroelectric phase has been selected in the selection period (T.sub.12 in the figure) as shown in FIGS. 31A and 31B, the state of the antiferroelectric phase can be maintained regardless of the duration in which the bias voltage is applied. However, in further research by the inventors, a phenomenon was observed in several liquid crystal materials in which the state gradually changed from the antiferroelectric phase to the ferroelectric phase as time elapsed from when the bias voltage was first applied as shown in FIG. 31C. Causes for this are considered to be the occurrence of a pretrasitional effect in the low voltage range as shown in FIG. 33, and also an increase in the amplitude of the data signal superposed on the bias voltage during the non-selection period because V(A-F)S-V(A-F)t is large when the steepness of the threshold characteristic is low. Phenomena such as these cause such problems as a lower contrast ratio as the duty ratio of the element increases.
The other problem concerns the speed of relaxation from the ferroelectric phase to the antiferroelectric phase. The speed of the relaxation is slower than the speed of response in switching in the opposite direction. In addition, a temperature dependence is observed in the speed of relaxation. By means of the prior art driving method, the scanning frequency had to be set low to match the response characteristic of the liquid crystal material used, which did not allow smooth scrolling of screen or smooth movement of a pointing devices.
The invention solves the above problems and its purpose is to offer a multiplexing drive method that takes sufficient advantage of the features of the tristable switching.